The majority of combustible liquid fuel used in the world today is derived from crude oil. However, there are several limitations to using crude oil as a fuel source. For example, crude oil is in limited supply; it includes aromatic compounds believed to cause cancer and contains sulfur and nitrogen-containing compounds that can adversely affect the environment.
Alternative sources for developing combustible liquid fuel are desirable. An abundant source is natural gas. The conversion of natural gas to combustible liquid fuel typically involves converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. An advantage of using fuels prepared from syngas is that they typically do not contain appreciable amounts of nitrogen and sulfur and generally do not contain aromatic compounds. Accordingly, they have less health and environmental impact than conventional petroleum-based fuels. Fischer-Tropsch synthesis is a preferred means for converting syngas to higher molecular weight hydrocarbon products.
Fischer-Tropsch synthesis is often performed under conditions which produce a large quantity of C20+wax, which must be hydroprocessed to provide distillate fuels. Often, the wax is hydrocracked to reduce the chain length, and then hydrotreated to reduce oxygenates and olefins to paraffins. Although some catalysts have been developed with selectivity for longer chain hydrocarbons, the hydrocracking tends to reduce the chain length of all of the hydrocarbons in the feed. When the feed includes hydrocarbons that are already in a desired range, for example, the distillate fuel range, hydrocracking of these hydrocarbons is undesirable.
It would be advantageous to provide a method for hydroprocessing Fischer-Tropsch wax which minimizes the hydrocracking of hydrocarbons in the distillate fuel range. The present invention provides such methods.
U.S. Pat. No. 6,583,186 discloses a means of hydroprocessing Fischer-Tropsch products without overcracking distillate components. In this scheme, however, all of the heavier, hydrocracked material is recombined with lighter materials, such as Fischer-Tropsch condensate and subsequently hydrotreated. There is no interstage separation step following hydrocracking, as in the instant invention.
U.S. Pat. No. 6,224,747 discloses hydrocracking a VGO stream in a hydrocracking reaction zone within an integrated hydroconversion process. Effluent from the hydrocracking reaction zone is combined, without interstage separation with a light aromatic-containing feed stream, and the blended stream is hydrotreated in a hydrotreating reaction zone. The hydrocracked effluent serves as a heat sink for the hydrotreating reaction zone. The integrated reaction system provides a single hydrogen supply and recirculation system for use in two reaction systems. This patent is not directed to hydroprocessing of Fischer-Tropsch products, as is the instant invention. Furthermore, there is no interstage separation between the hydrocracking and hydrotreating stages.